In gas-assisted injection molding of hollow tubular plastic parts, there are basically two types of processes, namely, gas through the injection nozzle and gas through a runner or the mold cavity. In both processes, the molten plastic separates into two flows in the mold cavity that ultimately join together at some point along the cavity where they start fusing together at their interface or weld line. The amount of molten plastic injected is a prescribed amount less than the volume of the mold cavity as determined by the desired wall thickness and is commonly referred to as a "short shot". A prescribed amount of inert gas under pressure is either simultaneously or subsequently introduced into the center of the molten plastic. The combination of high melt surface tension of the molten plastic at the cavity surface and the lower viscosity of the inwardly located hotter molten plastic confines the gas to form a hollow in the latter region. The molten plastic that is displaced by the gas is pushed into the extremities of the mold cavity, packing out the mold part and forming a void extending to opposite sides of the interface where the two plastic flows have joined and are fusing together to eventually form a solid weld section. The gas pressure is then normally maintained or held while the plastic proceeds to cure to form the finished part. Examples of such methods and apparatus are disclosed in U.S. Pat. Nos. 4,106,887, 5,098,637, 5,028,377, 5,204,051 and 5,198,238 and U.S. Ser. No. 07/955,767, filed Oct. 2, 1992 and assigned to the assignee of this invention.
As compared to conventional injection molding, the gas assist offers the advantages of no molded-in stresses, lower tool cost, reduced clamping forces, significant reduction in sink marks and improved surface finish. However, the strength of the weld line is relatively weak because of the high viscosity of the plastic melt at the front of the flows and because of the lack of interaction of the plastic molecules at the resulting stationary interface where the two molten plastic flow fronts meet. As a result, there is little, if any, polymer molecule mechanical entanglement or poor alignment of reinforcing fibers. Accordingly, there is optimum reinforcing fiber orientation in a melt/solid weld section encompassing the weld line that could enhance the weld strength. Thus, the molded hollow tubular part is inclined to breakage at the weld section.